An air pollution control device is used in thermal power plants and chemical plants. In the air pollution control device, a denitration device, an air preheating unit air heater, a heat recovery unit of a reheating heat exchanger (gas-gas heater), a dry electronic precipitator, a wet desulfurization device, a reheater of the reheating heat exchanger, and a stack are sequentially provided from the upstream side toward the downstream side of a flue gas flow path. A gas-gas heater including a preheating unit that is provided on a flue gas introduction portion of a reheater and that preheats flue gas introduced into the reheater body has been developed as the heat exchanger used in the air pollution control device such as the above (for example, see Patent Literature 1). In the gas-gas heater disclosed in Patent Literature 1, wet flue gas that has passed through the wet desulfurization device is preheated and dried in the preheating unit supplied with a heat medium having been heated by the heat recovery unit and the heating unit. Because the flue gas is dried, it is possible to reduce dust in the flue gas from adhering to the inside of the reheater body and reduce corrosion inside of the reheater body resulting from wet components in the flue gas.